Author Archives: Paul Taylor

Dummy loads for an AM PWM transmitter build

My Pulse Width Modulated AM transmitter project required dummy loads to match the load impedance of the RF power stage (50 ohms) and the Pulse Width Modulator (12 to 16 ohms). Surely any self-respecting amateur radio station would have a decent 50 ohm dummy load, I hear you ask incredulously. I do have one at VK3HN … or I did.

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A 300 Watt DC Power Supply for a QRO AM Transmitter

This post describes construction of a 300 watt DC power supply for use in a 150 watt Class D AM transmitter. The transmitter design calls for about 150 volts DC at 2 amps continuous and up to 4 amps peak. This post describes the 240v mains DC supply. The switch mode regulator is described in another post. The other modules in this QRO transmitter project are to follow.

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A 300w switching regulator for a QRO AM transmitter power supply

Laurie VK3SJ is an accomplished RF designer and homebrewer in Melbourne’s 160m scene. You can hear his excellent AM signal regularly on the 160m AM Coffee Break net most weekdays. Laurie has spent a great deal of time experimenting with Class D, E and Pulse Width Modulators. He has an interesting Class D AM transmitter design with a pulse width modulator. Laurie’s design has been built by Wayne VK3ALK, who I met online in the Class E Forum on AMFone. After lots of emails and a few calls with these homebrew AM experts, I decided to proceed with my build of a VK3SJ AM transmitter for 160m.

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A Pulse Width Modulator for the FAT5 160M Class E Transmitter

After a few months of operating my FAT5 160m AM Class E transmitter with its linear modulator, it was time to try Dave and Eric’s Pulse Width Modular module (PuWMa). This is a PWM using the Texas Instruments’ UCC25701 PWM chip, with TC4421 drivers and IRF640 switching FETs, followed by the obligatory Low Pass Filter to clean the 170kHz AC off the modulated DC.

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Microphone Compressor from 1978 lives again (ETI490)

Whilst sorting electronics junk recently I unearthed this board. My build of the Electronics Today International microphone compressor, ETI490, published in the magazine in December 1978. I remember preserving my copy of this magazine for many years, long after the pages faded and cover tattered, as it contained the construction article for this useful amateur radio station component.

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Summit Prowler 6: A pocket-sized SSB/CW transceiver for 80, 40, 30 and 20m

Peter DK7IH is a master of compact homebrew transceivers. I’ve been reading his excellent blog where he chronicles more than half a dozen homebrew HF transceivers and related projects. Most are compact SSB superhet transceivers with digital VFOs, AGC, metering and PAs in the range 5 to 50 watts. A few of these rigs are just so remarkably tiny I wonder how he has the persistence and patience, not to mention how he gets them stable. With Peter’s fine examples in my head, I started daydreaming about a compact SSB/CW transceiver, hoping to go a fair bit smaller than my slightly chunky attempt at a hand-held from last year.

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160m 100W Class-E AM transmitter (FAT5)

The FAT5 project is a 100 watt Class E AM transmitter from Dave GW4GTE and Eric GW8LJJ, dating back to 2011. The name derives from Dave’s desire to design a 100 watt AM workhorse using contemporary FETs to rival the ubiquitous AT5, a widely used war surplus boat anchor for low band AM.

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Mt Hotham SOTA Weekend Feb 2-3 2019

A brief post to share a short video from last week’s Mt Hotham SOTA weekend. Once again, Brian VK3BCM hosted several dozen SOTA and mountaineering enthusiasts at Anton Huette Lodge, Mt Hotham, for two days of summit bagging and SOTA talk. On the way up I activated the two ‘easy’ summits on the Mt Buffalo plateau, The Hump and The Horn.

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Homebrew 50 watt amplifier for 80-17m (W6JL)

Don W6JL’s 50 watt FET amplifier is a popular afterburner for FT817s and other QRP rigs and exciters. Don won the QST Homebrew contest in 2009 with this design. The amp offers a useful order of magnitude (12dB) power lift over QRP levels, and apart from the power FETs can be built from an averagely stocked junk box. The FETs are commonplace and cheap — they can be bought for as little as 98c each!

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